Here is the item again:
With the huge number of confusing rules colliding in her
head, (A)
Cindy took the quiz on fragments
(B) and
hoped, (C)
that she would remember enough of the material to pass.
You correctly realized that the comma after hoped caused an error. That she would remember ... is a subordinate clause, so it requires no comma to connect it to the main clause in front.
To fix this problem, you chose to do this:
With the huge number of confusing rules colliding in her head, Cindy took
the quiz on fragments and hoped that she
would remember enough of the material to pass.
You have correctly employed Punctuation Rule 1:
Main clause + Ø + subordinate clause.
Toot away!